Abstract: The present invention relates to a method of determining P-glycoprotein expression and/or function for a patient with solid tumors, leukemias, and other malignancies. The invention also relates to using a P-glycoprotein expression and/or function diagnostic in conjunction with methods for treating solid tumors, leukemias, and other malignancies with a chemotherapeutic agent in combination with zosuquidar. The methods are particularly effective in treating acute myelogenous leukemia, metastatic breast cancer, and other cancers expressing P-glycoprotein, wherein the P-glycoprotein expression and/or function is used to select a treatment option for the patient.